“These cover a broad scope of patients with metastatic breast cancer and offer new options,” said Jose Baselga of the Harvard Medical School, who was involved in both studies. “Together, I think these will be practice-changing. I think it’s exciting.”

In one of the studies, Baselga and his colleagues studied 808 patients with advanced breast cancer whose tumors were positive for the so-called HER-2 receptor. About 20 percent of breast cancers are HER-2 positive.

The women received the standard chemotherapy drug docetaxel plus the newer drug Herceptin, which only helps women whose tumors are HER-2 positive, along with either a placebo or a newer drug called pertuzumab, which also interferes with HER-2.

The tumors among the women who got pertuzumab did not progress for 18.5 months, compared to only 12.4 months for those who got the placebo.

“An improvement of six months is a big deal in this patient population,” Baselga said in a telephone interview.

Baselga acknowledged that the key measure is whether a drug actually helps patients survive longer. But he was optimistic that would be the case based on these findings. There were 69 deaths among the 402 women who got pertuzumab, compared with 96 among the 406 patients who got the placebo.

“It’s too early to say, but I think it’s likely this will show a survival benefit,” he said, noting that so far the drug appears to be safe and produced minimal side effects, such as diarrhea.

The study was sponsored by Genentech, which makes pertuzumab.

In the second study, researchers gave 724 postmenopausal women whose tumors responded to the hormone estrogen either the drug exemestane, which is already used, plus a placebo, or exemestane plus a drug called everolimus. Everolimus, first developed as an anti-rejection drug, has already been approved to treat other forms of cancer, such as kidney cancer.

Among the 239 women who got only exemestane and a placebo, they experienced no growth of their tumors for about 3.2 months. For those 485 who got everolimus, the tumors did not progress for 7.4 months.

Although the difference is relatively small, Baselga said it was significant.

“This is one of the most positive studies in breast cancer in a patient population whose options are quite limited,” he said.

The study was sponsored by Novartis, which makes everolimus.

Other experts said the findings were promising, but additional work was needed to see if they translated into longer survival.

“Will these advances translate into survival differences? Only time will tell on that one,” said Larry Norton of the Memorial Sloan-Kettering Cancer Center. “But keeping symptomatic or potentially symptomatic cancer at bay for months is still, in my mind, a worthwhile achievement, something our patients want and need while we search for a cure.”

Others, however, said it was too soon to conclude the new regimens should be adopted.

“These results are promising but should not change practice,”said Fran Visco of the National Breast Cancer Coalition, an advocacy group. “We need to wait for the survival data. We should not be celebrating differences in tumor growth that have no real clinical impact. Women deserve drugs that extend life with minimal side effects.”